Gas is a valuable natural resource and it is of interest to detect leakage of gas both from an economic aspect as well as for the hazard a leaking gas may present to its surroundings.
Current gas detection methods may for instance depend on absorption of e.g. wavelengths of electromagnetic radiation, with different gases having different absorption spectra, on chemical reactions, changes in electrical conductivity or in the capacitance of thin film.
A drawback with these methods is that the kinetic of gas absorption is quite slow, thereby limiting the field of application of these absorption methods.
Faster methods are based on the physical properties of various gases. Thermal conductivity is used for gas sensing in chromatography and optical absorption in the infrared region of the spectrum is used for carbon dioxide detection. For hydrogen detection there are methods that use catalysts. However, in contact with other gases the catalyst can be poisoned and the response of the detector is dramatically reduced.
Some known methods utilize the phenomenon of resonance in a resonance cavity to detect the presence of a gas in the resonance cavity. Resonance occurs when a distance d between a reflecting wall and e.g. an oscillator that transmits waves in the cavity, is an integer of half wavelengths, i.e.
                    d        =                  n          ⁢                      λ            2                                              (        1        )            where n=1, 2, 3 . . . . The wavelength of a vibration in a gas is dependent of the gas because the sound velocity is different in different gases, the wavelength being given by the relation:
                    λ        =                  v          f                                    (        2        )            where v is the sound velocity and f is the vibration frequency.
An ultrasonic method utilizing resonance has been described in the article “A gas analysis instrument based on sound velocity measurements” by E. Griffiths published in the Proceedings of the Physical Society, 1926. The article describes a quartz crystal and a reflecting wall parallel to the quartz crystal, which together form a resonance cavity. The quartz crystal can generate mechanical longitudinal waves in the cavity, with a 40 kHz frequency. Resonance in the cavity is detected by an increase in the anode current of an oscillating circuit which comprises the quartz crystal. By measuring the distance between adjacent resonances, i.e. a half wavelength, sound velocity in different gases is determined as well as the concentration of a gas in a gas mixture.
However, none of the above presented methods can be used for measurements of low gas concentration.